When I was a young Bad Astronomer, one of my favorite night-sky targets for my telescope was the globular cluster M5, an easy-to-spot fuzzy jewel in the southern sky. Over the years I must have taken a look at it a hundred times, my telescope barely resolving a few of the brighter stars in it.

M5 is a collection of at least 100,000 stars, all orbiting each other like bees around a beehive, held together by their mutual gravity. It’s located about 25,000 light years away, and is something like 150 light years across. It’s one of more than 150 such clusters of stars orbiting our Milky Way galaxy.

And it’s old: it’s probably been around for 12 billion years. Yikes. I hope I look as good when I’m that relatively scaled age.

I don’t have much to add here; I’ve written about globulars many times (see Related Posts below), so instead I’ll simply let you soak in its beauty… and note that despite my best efforts, I couldn’t seem to work in an Ultimate Computer reference in this post’s title. You may pull out the plug, Mr. Spock!

I imported the 3150×3150 image. Inverted the color and printed on 8.5×11 paper. I noticed that the small stars do not appear to be randomly distributed, even accounting for the central dense area. The small stars seem to be clustered in chains of 3-10 stars.

Are my eyes playing tricks? Or, is this some gravitational lens effect?

IIRC, someone hypothesized that this puppy has a black hole in the center holding it all together.
Way cool!
I showed M13 to people at Fan Mountain’s Open House Friday, then took an image of it afterward. I still have to stack & dark subtract the images… Must get busy!

7. Molly Says:
May 2nd, 2011 at 2:27 pm
What’s the difference between a star cluster and a galaxy?
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Good question! And, as it turns out, there isn’t an official answer. Colloquially, cluster usually refers to a few hundred stars and galaxy refers to millions or billions, but there’s no standardly-recognized dividing line.

In some recent journal papers, a few astronomers have tried to propose standardized definitions based on size, presence of dark matter, occurrence of star collisions, and other factors. Nailing down a definition will probably be as contentious as defining a planet.

In reality, a galaxy is just a very, very big star cluster. But astronomers and backyard observers will call a star cluster just a grouping of stars within or in close orbit around a galaxy.

Star clusters tend to be grouped (ha ha) in two catetogeries:

A globular cluster (like M5 referenced in this post) is several hundred thousands of stars and orbit our galaxy. These could be the leftovers of a galaxy eaten by our Milky Way galaxy.

An open cluster can range from tens of stars to thousands of stars. This is usually a group of stars that were born in the same nebula and are now spreading out. There are many examples. Some are tight-knit, while others have spread out pretty well. As we get into summer, there are two nice ones between Scorpius and Sagittarius that you can see with binoculars. Also, the main stars of the Big Dipper (except for the tip of the handle and the tip of the bowl) are actually part of a very open star cluster.

Globulars are definetly my favorite target in the night sky. Somehow they don’t look like that through my 8 inch Newtonian though. Fantastic picture. I guess I need a bigger telescope and a little more altitude…

But… 100,000 stars and just 150 lightyears across… My skill at math fails me completely, but I’m curious about one thing – what’s the mean distance between any two stars in a cluster like this? Has anyone figured it out? I’m trying to imagine what the view would be like from deep within the cluster.

Not really great with numbers…but here it is: Sphere 150 ly dia and 100,000 stars distributed evenly within would give you about one star for each 70 cubic light years, which means that the average distance between stars would be about 2.5 light years.

What’s it gonna look like? Alpha Centauri (chosen because it is almost a twin of the Sun in many respects) the closest bright star is about 4.3 light years away. A Centauri is about as bright as each of the stars of the Big Dipper. If A Centauri were only 2.5 light years distant, it would probably be the brightest thing in the sky except for the Sun and the Moon. Brighter than Sirius, you couldn’t miss it.

If you were in the center of the M5 cluster, the night sky would be filled with thousands of first magnitude stars, and even brighter, and a plenitude of lesser lights. Apparent average distance (as your eye perceives) between the stars would be less than the moon’s disc. There would probably be minimal grouping or none at all of stars into visible constellations, there would simply be too many stars.

On a clear day, the sky would also be filled with many, perhaps dozens of day-visible stars, and at night you wouldn’t even need street lights to get around. Even a full moon would not be enough to equal the lights of the stars in the skies.

If you’ve ever climbed a tree or stood beneath one filled with fireflies…except for the color, that’s pretty much the effect.

I did the same calculation as Jockaira and agree with the 2.5 light-year average distance. It’s surprising it’s that much, considering that our nearest neighbor is only 4.3 ly. Does that imply that our galactic neighborhood is only (4.3/2.5)^3 =5 times less dense than a galactic cluster?

I suppose that if you were at M5 looking in our direction, you’d be looking at an average density arm of the Milky Way and it would look denser than we imagine space in our vicinity to be.

Also quoting the eponymous Stanley Kubrick movie based on the novel* where a version of that scene takes place too – except with the “star gate” monolith there located in orbit around Jupiter rather than on Japetus (now Iapetus) a Moon of Saturn instead.

Of course, there’s also the issue of how many multiple & binary stars exist in globulars to consider there too.

Wonderful image of Messier 5 there – so beautiful. Love it.

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* In fact , it’s more complicated than just that – Clarke’s 2001 : Space Odyssey book was apparently based on Kubrick’s movie as much as vice-versa with the two developing in parallel and interacting with each other.

@22. Joseph G :” Also, are the colors in that image false-color, or do they correspond, roughly at least, to the colors of those stars?”

From the link in the Bad Astronomer’s third sentence immediately above the image itself :

The picture was created from images taken through a blue filter (F435W, coloured blue), a red filter (F625W, coloured green) and a near-infrared filter (F814W, coloured red).

It would seem that this is a “false colour” image – unless you can see in the near-infrared!

Still not sure I grok the distinction between dwarf galaxies and globular clusters.

Sometimes as in the case of Omega Centauri, these objects can be the one & the same thing at different times.

Omega Centuari is a globular orbiting our Milky Way (now) which is thought to have begun its life as a dwarf galaxy in its own right or so I gather. So these categories may, at least to some extent, be interchangeable depending on where the huge globe of stars happens to be at the time. Incidentally, Kaptetyn’s Star a nearby red dwarf with a high proper motion (apparent movement across our sky) is thought to be a former member of Omega Cen stripped away at some later stage.

I guess as a rule of thumb : If it orbits close to another galaxy – effectively as part of that other galaxy’s halo region – and has no globular clusters of its own then its a globular cluster. If it doesn’t orbit another galaxy directly or is a very lo-oong way from any other “island universe”* and especially if it boasts globular clusters of its own then its a dwarf galaxy. Hope that helps.

* Or more likely metal-poor sub-dwarfs because of the typically *much* lower metallicity (presence of elements beyond hydrogen, helium and lithium) in globular clusters.

+ G type yellow dwarfs from G0 to G7 are probably old enough to have started evolving off the main-sequence. (ie. stopped core hydrogen fusion) But no G8 or cooler dwarf or sub-dwarf star has yet had enough time to cease core hydrogen burning and evolve into gianthood yet. The cosmos just isn’t old enough.
– Source : James B. Kaler, ‘The Hundred Greatest Stars’, Copernicus Books, 2002.

It makes me think that should anyone live in the midst of it, it would be too bright for them to know that they are part of a galaxy that is part of universe of galaxies. Like the critters under the Europan ice who don’t know they are just on a moon.

We are very lucky to be on the surface of a planet with frequent clear skies in the middle of galactic nowhere.